The invention described below relates to the use of a micro-machined mirror for optical beam steering, switching, and scanning. More specifically, the improvements relate to enhancements to the mirror with respect to its reliability as well as its reflectivity.
In the prior art, micro-machined mirror reflectors have been described to comprise a single layer of gold affixed to a polysilicon mirror plate by an adhesion layer of chromium. Gold is a suitable choice because it is compatible with surface micro-machining processes which can be used to fabricate the mirror, is easy to pattern, and can be highly reflective (xcx9c95%). However, a layer of gold by itself cannot provide reflectivity significantly greater than 95%, is not adequately reflective at wavelengths shorter than about 600 nm, and if unprotected by appropriate coating layers is susceptible to corrosion upon long-term exposure to certain operating environments.
Gold (Au) does not adhere well to polysilicon without the inclusion of an adhesion layer, for example, chromium (Cr). Gold is a relatively inert material but does have the capacity to react in certain environments, especially those containing trace amounts of chlorine compounds. In reacting with chlorine, gold will corrode to render it unsuitable as a reflector. Although such a reaction may take years to have any significant impact, it may present a potential long-term reliability problem. In addition, pinholes in the gold layer may allow access to the underlying chromium adhesion layer, which can also corrode in the presence of chlorine compounds. Corrosion of the chromium may result in the loss of adhesion and delamination of the gold layer.
In 1980, K. E. Petersen described a silicon torsional scanning mirror (xe2x80x9cSilicon Torsional Scanning Mirrorxe2x80x9d, IBM J. Res. Develop., V24, Sep. 5, 1980, pp. 631-637) that included reflective surface made from a single layer of aluminum deposited on a rotatable silicon mirror plate. For this application, the versatility of micro-machining, aluminum was a suitable choice because it was easily incorporated into the process for creating the mirror.
More demanding applications require increased attention to the design of the reflective layer. In 1992, U. Breng, et. al., reviewed various metal films with respect to their reflectivity at an incident light wavelength of 780 nm (xe2x80x9cElectrostatic Micromechanic Actuators,xe2x80x9d J. Micromech. Microeng., V2, 1992, pp. 256-261). Fabrication and operation of Breng""s micro-machined mirror was quite similar to that of Petersen""s mirror. But rather than settle on aluminum for the reflective layer, these researchers noted that higher reflectivity could be obtained by using a silver or gold. They chose to use gold as their reflector due to the long-term environmental instability of silver.
Multi-layer coatings have been studied as a solution for improving reflectivity from an optical surface. For example, through careful selection of the materials and thicknesses, the coatings can be targeted for very high reflectivity at the particular wavelength(s) of interest. Furthermore, with attention paid towards their chemical properties, the coatings can be made compatible with the chemistry of the in-use environment. However, in general, each layer of a multi-layer coating is likely to be deposited with significant thin film stress. When deposited on a very thin (2 um) free-standing plate, thin film stress can cause distortions in the plate. Minimization of the mirror plate distortion, both as fabricated and with respect to ambient temperature fluctuations, is necessary for good optical performance. In addition, the materials used in the coating should be compatible with the chemicals used in the fabrication process.
A widely used method to improve surface reflectivity from a single thin metallic layer employs a quarter wave stack (see, for example, H. A. MacLeod, Thin Film Optical Filters, McGraw-Hill, 1989). A quarter wave stack may be comprised of alternating layers of two non-absorbing dielectric materials one of which has a xe2x80x9chighxe2x80x9d index of refraction; the other has a xe2x80x9clowxe2x80x9d index. The thickness of each layer is chosen such that it""s optical path length (index of refraction times physical film thickness) is one quarter of the wavelength (xcex/4) of the light of interest. As the number of high-low (HL) pairs deposited on a given surface increases, the reflectivity for light incident on the surface at this wavelength increases until a theoretical maximum of somewhat less than 100% can be achieved. It should be noted that these examples are relatively simple optical coatings. A multi-layer coating can become quite complex in terms of the various films and physical film thicknesses and is determined by the function (i.e. beam-splitting, phase retardation, anti-reflection, etc.) that it is called upon to perform.
Multi-layer coatings such as those described above have been employed only rarely within the field of micro-machining. A known example was described by Jerman, et. al. (xe2x80x9cA miniature Fabry-Perot interferometer with a corrugated silicon diaphragm support,xe2x80x9d Sensors and Actuators A, V29, 1991, pp. 151-158). A Fabry-Perot interferometer is an optical element consisting of two partially reflecting low-loss mirrors separated by a gap. In Jermans""s device, multi-layer dielectric coatings were used as mirrors and were designed to be highly reflective at wavelengths between 1.30 and 1.55 um. Micro-machining techniques were employed to miniaturize the interferometer and to provide the gap as well as the ability to vary the gap width.
The optical coating layers described above exhibit some amount of residual thin film stress. In micro-machining, the coating may be deposited onto very thin structures that upon release from the substrate during manufacture can deform as a result of the residual stress in the coating. In addition, if the coatings are metallic, the coatings may form a bimetallic element, wherein additional deformation will be induced in the thin structures as a function of ambient temperature due to the differing coefficients of thermal expansion between the metal and the microstructure material. Attempts to minimize these deformations have been made by depositing the metalized reflector on both sides of the micro-machined structures so as to balance the stress with respect to the neutral axis of the structure. However, due to difficulties in controlling the metallic layer properties, the flatness of the plates that has been achieved has been less than optimal. In xe2x80x9cMultilayer Film Structure and Vertical Cavity Surface Emitting Lasersxe2x80x9d (U.S. Pat. No. 5,729,567, March 1998), S. Nakagawa addressed the similar problem with respect to a multi-layer optical coating on GaAs substrates. Here a quarter wave stack comprised of silicon dioxide (L) and titanium dioxide (H) was used on a mirror. Deposition conditions were developed that provided film stresses for the two materials of equal magnitude but opposite polarity, such that the HL pair reduced stress.
Taking into consideration the discussion above, it is clear that improvements are needed in the prior art. Some of these improvements are discussed below.